Gaseous effluxes from soil are indicators of biological processes that are occurring below ground and if quantified the relative importance of particular processes to the whole ecosystem can be determined. For example, the efflux of CO2 out of soil, commonly referred to as soil respiration, comprises the respiration of plant roots and the respiration of soil heterotrophs (bacteria, fungi, arthropods). Soil respiration is a very large fraction of gross primary productivity, and its quantification is a high priority in attempts to establish ecosystem carbon budgets. The current emphasis on ecosystem management for increased carbon sequestration mandates improved monitoring of soil respiration. If the carbon flux from the soil cannot be accurately measured, it cannot be assumed that there is increased storage of carbon in long-term soil pools. The importance of quantifying rates of CO2 efflux from soils was recently accentuated by the European Science Foundation""s workshop this summer that addressed the problems associated with measurements of soil respiration.
The primary problem facing strategies for measurement of soil respiration is the tremendous spatial and temporal heterogeneity in the rates of soil CO2 efflux. CO2 efflux varies seasonally and over the course of a day, mostly in response to root growth phenology and to changing temperature. Currently, this variation is handled by establishing temperature response relationships that can be applied to point-in-time measurements of efflux and continuously monitored soil temperature. The correction factor can be so large, however, that this approach may introduce errors. This is further exacerbated by the inability to accurately assess the phenology and magnitude of root growth below ground. Spatial heterogeneity in soil CO2 efflux can be handled by making many replicate measurements, but because each measurement takes some time, the spatial variation becomes confounded with temporal variation. Most of these same problems face researchers who attempt to measure efflux of other greenhouse gases, such as CH4, N2O, N2, and NO2. Instrumentation currently available for continuously measuring soil gas efflux do not solve these problems because techniques used for trapping the gases alter the microclimate of the soil and therefore alter the rates of gas efflux. The chamber in this invention used in conjunction with automated switching and analysis can solve the problems associated with temporal variability.
U.S. Pat. No. 5,355,739 to Cooper et al. is representative of sampling chambers that prevent natural metabolic processes and thus influence soil gas efflux rates. This point-in-time measuring device, especially designed for spot measurements of gas emissions, is not suitable for continuously monitoring gas emissions from soil or even from landfills because it does not permit the natural drying and wetting of the soil or landfill when left in place over long time periods.
The criteria for perfecting an automated soil gas efflux system with the capability to monitor rates continuously over long time periods revolve primarily around the development of an appropriate chamber for trapping the gases as they evolve from the soil without disturbance to the soil and to the natural microclimate within the chamber. The gases evolving from the soil must be moved with the ambient air stream at constant and quantifiable flow rates sequentially through many such chambers inverted over the soil surface to an appropriate analysis system. The analysis system must accurately monitor concentration differences of the gases in the air before entering (reference air) and after exiting the chambers (sample air). Furthermore, the air moving through the chamber should not disturb the natural air boundary layer at the soil surface more than might occur at average wind speeds. The air stream should not alter the atmospheric pressure within the chamber.
Continuous observations of soil gas efflux when sustained over a range of environmental and growth conditions allow for the quantification of component processes contributing to whole ecosystem soil gas flux. Instrumentation currently available for continuously measuring soil gas efflux do not solve these problems because techniques used for trapping the gases alter the microclimate of the soil and therefore alter the rates of gas efflux.
A chamber for trapping soil gases as they evolve from the soil without disturbance to the soil and to the natural microclimate within the chamber has been invented. The chamber does not alter the metabolic processes that influence soil gas efflux rates. A multiple chamber system provides for repetitive multi-point sampling, undisturbed metabolic soil processes between sampling, and an essentially airtight seal around the chamber housing during sampling. The chamber housing operates at essentially ambient atmospheric pressure during sampling.
The chamber operates by closing over the soil in response to a computer signal and remains closed for a preselected time period, preferably a 14 minute period, before opening again. By being closed only periodically, the chamber allows normal drying and wetting of the soil between measurements. After testing a single prototype chamber, seven additional chambers were built and an automated switching system was purchased and programed to sequentially open and close the chambers in concert with an automated infrared gas analysis system. Soil respiration rates measured with the automated chambers were in agreement with proven point-in-time measurements and have been run for several months without altering the soil microclimate.